Polymeric structural panel

ABSTRACT

A polymeric twin wall board which includes a first substantially planar sheet forming the upper side of the board, a second substantially planar sheet forming the underside of the board and a plurality of spaced longitudinally extending webs connecting the first substantially planar sheet to the second substantially planar sheet wherein the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs is no more than about 1% of the average distance between adjacent longitudinally extending webs.

[0001] This invention concerns a lightweight polymeric structural panel.It particularly concerns an im proved twin wall board of the type whichincorporates two substantially planar sheets spaced apart from eachother and connected by a plurality of transverse webs.

[0002] Polymeric twin wall panels have been manufactured and used formany years. Such panels have broad application for packaging, signage,building and for other similar purposes. These products have foundfavour as alternatives to twin wall cardboard products as they are moredurable, are not susceptible to water damage and are generally strongerthan cardboard products of similar weight. Notwithstanding theadvantages of polymeric twin wall products in terms of physicalperformance there are limitations in using these boards as signs or aspackaging due to difficulties in achieving high print quality on thepolymeric surface. This has restricted the use of these products in somecircumstances.

[0003] It is an object of the present invention to provide a polymerictwin wall product which is better suited to receiving and displayingprinted words and graphics than existing polymeric twin wall products.

[0004] The applicants have ascertained that one factor relevant to thesuitability of polymeric panels for receiving and displaying printedmaterial at high resolution is the level of the intended print surfaceat the junctures between the respective transverse webs compared withthe portions of the print surface intermediate adjacent transverse webs.This is particularly so where the distance between adjacent transversewebs is small.

[0005] In accordance with a first aspect of the present invention thereis provided a polymeric twin wall board which includes a firstsubstantially planar sheet forming the upper side of the board, a secondsubstantially planar sheet forming the under side of the board and aplurality of spaced longitudinally extending webs connecting the firstsubstantially planar sheet to the second substantially planar sheetwherein the difference between the level of the top surface of the firstsubstantially planar sheet adjacent to its juncture with any of thelongitudinally extending webs and the level of the top surface of thefirst substantially planar sheet intermediate any of the adjacentlongitudinally extending webs is no more than about 1% of the averagedistance between adjacent longitudinally extending webs. Most desirablythe difference is no more than about 0.5%. Preferably the longitudinallyextending webs are equally spaced and the difference in levels is nomore than 1% of the distance between adjacent longitudinally extendingwebs.

[0006] Ordinarily, it is also desirable that the difference between thelevel of the outer surface of the second substantially planar sheetadjacent to its juncture with any of the longitudinally extending websand the level of the top surface of the second substantially planarsheet intermediate any of the adjacent webs is no more than about 1% ofthe average distance between adjacent longitudinally extending webs.Again, most desirably the difference is no more than about 0.5%. Thisavoids the board having alternative sides which are differently suitedfor receiving printed material.

[0007] Preferably, the twin wall board of the invention includeslongitudinally extending webs which are separated by a distance ofbetween 2.0 to 5.0 mm. In boards made to these preferred dimensions itis preferred that the difference between the level of the top surface ofthe first substantially planar sheet adjacent to its juncture with anyof the longitudinally extending webs and the level of the top surface ofthe first substantially planar sheet intermediate any of the adjacentlongitudinally extending webs is no more than 0.020 mm. Most preferablyit is no more than 0.010 mm. Desirably, the profile of the secondsubstantially planar sheet is the same. The thickness of the webs andthe first and second planar sheets can vary depending on the strengthrequirements of the intended board. However, generally these walls ofthe board are of about equal thickness and between 0.1 to 0.3 mm.

[0008] The polymeric material utilized for the manufacture of the boardsof the invention can be of any type as known in the art. Preferably, theboards are made from a modified or unmodified polyolefin. Mostpreferably, the boards are manufactured from a homopolymer or co-polymerof polypropylene or a high density polyethylene material. It isdesirable that the polymeric material have good thermal conductivity.Preferably the thermal conductivity of the material utilised is greaterthan 0.20 W/m.K. Most preferably the polymeric material utilised has athermal conductivity of between 0.25 to 0.35 W/m.K.

[0009] Whatever polymeric material is used it is most desirable that itscrystalline freezing point is higher than that of a standardpolypropylene (i.e. above about 112° C.). Most preferably thecrystalline freezing point of the polymeric material is more than 120°C. and less than 150° C. The applicant has found that production ofboards of the present invention is assisted by utilising a materialhaving a higher crystalline freezing point than materials conventionallyused for manufacturing twin wall board. This is because the polymericproduct once extruded can be more readily cooled to a temperature atwhich it will be resistant to non-uniform shrinkage. The crystallinefreezing point of the polymer is best measured using a differentialscanning calorimeter.

[0010] In accordance with a further aspect of the present inventionthere is provided a process for the manufacture of a polymeric twin wallboard which includes extruding a molten polymeric material through a dieconfigured to produce a board having opposed first and secondsubstantially planar sheets separated by a plurality of transverselongitudinally extending webs and delivering the said extruded materialinto a cavity bounded by upper and lower platens where the extrudedmaterial is cooled to form a solidified board, wherein a sufficientlyhigh vacuum is applied to at least one side of the extruded polymericmaterial whilst in the cavity so that the difference between the levelof the top surface of the first substantially planar sheet adjacent toits juncture with any of the longitudinally extending webs and the levelof the top surface of the first substantially planar sheet intermediateany of the adjacent longitudinally extending webs in the solidifiedboard is no more than about 1% of the distance between adjacentlongitudinally extending webs. Preferably the vacuum applied is suchthat this difference is no more than about 0.5%. Furthermore it ispreferred that the vacuum be applied to both sides of the extrudedpolymeric material.

[0011] In accordance with a further aspect of the present inventionthere is provided a process for the manufacture of a polymeric twin wallboard which includes the steps detailed above but where the distancebetween adjacent webs is between 2.0 to 5.0 mm and the vacuum applied toat least one side of the extruded polymeric material is sufficientlyhigh so that the difference between the level of the top surface of thesubstantially planar sheet adjacent to its juncture with any of thelongitudinal extending webs and the level of the top surface of thefirst substantially planar sheet intermediate any of the adjacentlongitudinally extended webs in the solidified board is no more than0.020 mm. Preferably the vacuum applied is sufficiently high to preventthe difference being more than 0.010 mm. Most desirably the vacuum isapplied to both sides of the extruded polymeric material.

[0012] In accordance with either aspect of the present invention it ispreferred that the platens be substantially flat and that they be cooledso that the temperature of the surface of the platen is 7° C. or lower.Most desirably the operating temperature of the surface of the platen isbetween 2.0 to 6.0° C.

[0013] As indicated above it is desirable in accordance with the processof the present invention that one utilise a polymeric material which,once extruded, can be more readily cooled to a temperature at which itwill be resistant to non-uniform shrinkage. Accordingly, it is preferredthat the polymeric material have a crystalline freezing point which ishigher than that of a standard polypropylene. Most preferably thefreezing point of the polymeric material is more than 120° C. and lessthan 150° C. Most preferably the freezing point of the polymericmaterial is more than 125° C.

[0014] The polymeric material preferably utilised in the processes ofthe invention is also of the type as indicated above in relation to theproducts of the invention—i.e. it is preferred that the polymericmaterial used be either a modified or unmodified polyolefin and it isdesirable that the thermal conductivity of the material is greater than0.200 W/m.K. Most desirably the polymeric material has a thermalconductivity of between 0.25 to 0.35 W/m.K.

[0015] When utilising a polymeric material of the type and having thecharacteristics indicated above the applicants have found that theplaten vacuum settings should be set so that the vacuum applied is atleast −150 mbar. Most desirably the vacuum is set at between −175 mBarand −300 mBar.

[0016] Whilst different methods might be used to cool the platens theapplicants find that if the platens incorporate channels for deliveringcooled water through or adjacent to the platen this is a practical wayof cooling the platen to the desired temperature.

[0017] The molten polymeric material in its hollow profile form istherefore, in the preferred processes of the invention, drawn hardagainst the platens by the applied vacuum with enough force to ensurethat the molten plastic substantially takes the flat structure of theplatens. It is most desirable that the polymeric material have most, ifnot all, of the characteristics indicated above as being preferred. Thehigh crystalline freezing point of the raw material will allow thepolymer to solidify more quickly than a standard polymer thus allowingthe sheet to hold the shape of the platens before any significantshrinkage can occur.

[0018] The preferred thermal conductivity properties of the rawpolymeric material also assist this process as the use of a materialwhich has better thermal conductivity enables a greater rate of heattransfer from the body of the extrudate and particularly in the planarsheets to the platens. This enables one to solidify the whole profileearlier than would be possible with a standard polypropylene. The heattransfer is also more effective when the temperature of the surface ofthe platens is reduced to the levels indicated above as preferred.

[0019] Any one of these changes enhances the process but to obtain themaximum benefits of the invention it is desirable that all of thefeatures are incorporated so as to maximize the rate of heat transferfrom the extrudate. It is most desirable that the higher vacuum appliedbe constant and that the extrudate be held in place while thesolidifying process takes place. In preferred embodiments the processincludes transferring the solidified product into a cavity between afurther pair of platens where the vacuum applied is lower (in the orderof −30 to −50 mBar) where the product can reach equilibrium before beingfinished by cutting and packing.

[0020] An example of the twin wall board of the invention and itsprocess of manufacture is hereinafter described with reference to thefollowing drawings in which:

[0021]FIG. 1 is a schematic cross section through a standard twin wallboard;

[0022]FIG. 2 is a schematic cross section through a twin wall boardproduct of the invention;

[0023]FIG. 3 is a schematic representation of a molten polymer extrudedin accordance with the method of the invention in its hollow profileform located between respective upper and lower platens; and

[0024]FIG. 4 is a schematic representation of a die head adjacent upperand lower platens showing the extrusion of the polymeric material andits delivery between upper and lower platens for the formation of a twinwall board of the invention.

[0025] With reference to FIG. 1 there is shown the cross section througha standard twin wall board. The cross section is shown in an exaggeratedform to emphasise the corrugations that one finds in standard product.The twin wall board 1 includes a first or upper surface 2, a second orlower surface 3 and a plurality of transverse longitudinally extendingwebs 4. Webs 4 extend along the full length of the product and join theupper surface 2 to lower surface 3. In FIG. 1 the letter “A” denotes theamplitude of the “sink”. In conventional products the sink is in theorder of 0.05 mm.

[0026]FIG. 2 illustrates a product made in accordance with theinvention. It will be noted that the upper surface 2 a is substantiallyplanar as is lower surface 3 a. The amplitude of the sink is less than1% of the distance between adjacent longitudinally extending webs 4 awhich are equally spaced. In the product depicted the distance betweenlongitudinally extending webs 4 a is 3.2 mm and the sink amplitude istherefore less than 0.032 mm. Most preferably it is less than 0.010 mm(i.e. less than about 0.3% ).

[0027] The product shown in FIG. 2 is preferably made from a modifiedpolypropylene with a DSC crystalline freezing point of 125° C. orgreater. It preferably has a thermal conductivity of 0.30 W/m.K orgreater and the wall thicknesses of substantially planar sheets 2 a and3 a are about 0.2 mm with the longitudinally extending webs 4 a being inthe order of 0.15 to 0.20 mm in thickness.

[0028]FIG. 3 shows a molten polymeric extrudate 5 in its hollow profileform positioned between platens 6 and 7. The molten polymeric form 5 isproduced using standard technology known in the art. It requires themelting of polymeric material and its extrusion through a die which isconfigured to produce the hollow profile form as depicted in FIG. 3 withfirst substantially planar sheet 2 a and second substantially planarsheet 3 a separated by transverse webs 4 a. The modified polypropylenematerial preferred in the practice of the invention is preferablyextruded at temperatures ranging between 190 to 230° C.

[0029] The platens 6 and 7 are cooled by delivering chilled waterthrough conduits which run along or through the respective platens. Theplatens are metallic and the chilled water is found to be effective inlowering the temperature of the surface of the platens between which theextruded material 5 is positioned. When chilling the platens with waterit is desirable that the water be delivered at a temperature which isbelow 5.5° C. but above 2° C. If the temperature of the water is muchbelow 2° C. it will start to freeze and this will create difficulties inoperation. If liquids other than water are used lower temperatures ofcourse can be achieved with benefits for the method of the invention.The lower the temperature of the platens the greater will be the rate ofheat transfer from the body of the extrudate 5 to the platens 6 and 7.

[0030] The vacuum applied to the extrudate through platens 6 and 7 ispreferably about

[0031] −175 mBar. It is applied to both the upper planar sheet 2 a andthe lower planar sheet 3 a. Extrudate 5 is held between the platens forsufficient time for the polymeric material to solidify and thus allowthe formed twin wall board to hold the smooth and flat shape of theplatens.

[0032] In FIG. 4 one can see extrudate 5 leaving the die head and beingdelivered between platens 6 and 7. Optionally air-blades can be used todirect cool air against the surface of the extrudate 5 in the locationindicated in FIG. 4. By directing cool air towards the extrudate as itleaves the die it is possible to accelerate the cooling process.

[0033] Once the polymeric material has solidified it is desirable thatthe formed board is conveyed or delivered into a cavity between afurther set of platens (not shown in the drawings) where a reducedvacuum is applied for sufficient time for the material to reachsubstantial equilibrium. The formed board is thereafter finished byconventional means and packed for delivery to end users.

[0034] The applicants have found that twin wall board produced inaccordance with the invention provides a quality surface suitable forreceiving detailed graphics by means of conventional printingtechniques.

[0035] It will be appreciated to those skilled in the art that thevarious modifications to the products and processes hereinbeforedescribed can be made without departing from the spirit and ambit of thepresent invention as defined in the following claims.

The claims defining the invention are as follows:
 1. A polymeric twin wall board which includes a first substantially planar sheet forming the upper side of the board, a second substantially planar sheet forming the underside of the board and a plurality of spaced longitudinally extending webs connecting the first substantially planar sheet to the second substantially planar sheet wherein the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs is no more than about 0.1% of the average distance between adjacent longitudinally extending webs.
 2. A polymeric twin wall board as claimed in claim 1 wherein the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs is no more than about 0.5% of the average distance between adjacent longitudinally extending webs.
 3. A polymeric twin wall board as claimed in either one of claims 1 or 2 wherein the longitudinally extending webs are equally spaced.
 4. A polymeric twin wall board as claimed in any one of claims 1 to 3 wherein the difference between the level of the outer surface of the second substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the second substantially planar sheet intermediate any of the adjacent webs is no more than about 1% of the average distance between adjacent longitudinally extending webs.
 5. A polymeric twin wall board as claimed in claim 4 wherein the difference between the level of the outer surface of the second substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the second substantially planar sheet intermediate any of the adjacent webs is no more than about 0.5% of the average distance between adjacent longitudinally extending webs.
 6. A polymeric twin wall board as claimed in any one of the previous claims which includes longitudinally extending webs which are separated by a distance of between 2.0 to 5.0 mm.
 7. A polymeric twin wall board as claimed in claim 6 wherein the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs is no more than 0.020 mm.
 8. A polymeric twin wall board as claimed in claim 7 wherein the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs is no more than 0.010 mm.
 9. A polymeric twin wall board as claimed in any one of claims 1 to 8 wherein the first and second substantially planar sheets are of equal thickness and the thickness is between 0.1 to 0.3 mm.
 10. A polymeric twin wall board as claimed in any one of the previous claims made from a homopolymer or copolymer of polypropylene.
 11. A polymeric twin wall board as claimed in any one of the previous claims wherein the thermal conductivity of the polymer used for the manufacture of the board is greater than 0.20 W/m.K.
 12. A polymeric twin wall board as claimed in claim 11 wherein the thermal conductivity of the polymeric material is between 0.25 to 0.35 W/m.K.
 13. A polymeric twin wall board as claimed in any one of the previous claims wherein the polymeric material from which the board is made has a crystalline freezing point above 112° C.
 14. A polymeric twin wall board as claimed in claim 13 wherein the crystalline freezing point of the polymeric material is more than 120° C. and less than 150° C.
 15. A process for the manufacture of a polymeric twin wall board which includes, extruding a molten polymeric material through a die configured to produce a board having opposed first and second substantially planar sheets separated by a plurality of transverse longitudinally extending webs, and delivering the said extruded material into a cavity bounded by upper and lower platens where the extruded material is cooled to form a solidified board, wherein a sufficiently high vacuum is applied to at least one side of the extruded polymeric material whilst in the cavity so that the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extended webs in the solidified board is no more than about 1% of the distance between adjacent longitudinally extending webs.
 16. A process for the manufacture of a polymeric twin wall board as claimed in claim 15 wherein the vacuum applied to at least one side of the extruded polymeric material whilst in the cavity is sufficiently high so that the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs in the solidified board is no more than about 0.5% of the distance between adjacent longitudinally extending webs.
 17. A process as claimed in either one of claims 15 or 16 wherein a vacuum is applied to both sides of the extruded polymeric material.
 18. A process for the manufacture of a polymeric twin wall board which includes, extruding a molten polymeric material through a die configured to produce a board having opposed first and second substantially planar sheets separated by a plurality of transverse longitudinally extending webs where the distance between adjacent webs is between 2.0 to 5.0 mm and delivering the said extruded material into a cavity bounded by upper and lower platens where the extruded material is cooled to form a solidified board, wherein a sufficiently high vacuum is applied to at least one side of the extruded polymeric material whilst in the cavity so that the difference between the level of the top surface of the first substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs in the solidified board is no more than about 0.020 mm.
 19. A process for the manufacture of a polymeric twin wall board as claimed in claim 18 wherein the difference between the level of the top surface of the substantially planar sheet adjacent to its juncture with any of the longitudinally extending webs and the level of the top surface of the first substantially planar sheet intermediate any of the adjacent longitudinally extending webs in the solidified board is no more than 0.010 mm.
 20. A process for the manufacture of a polymeric twin wall board as claimed in any one of claims 15 to 19 wherein the upper and lower platens are substantially flat and are cooled so that the temperature of the surface of the platens is 7° C. or lower at the time of delivering the said extruded material into the cavity bounded by the said platens.
 21. A process for the manufacture of a polymeric twin wall board as claimed in claim 20 wherein the temperature of the surface of both platens is between 2.0 to 6.0° C. at the time of delivering the said extruded material into the cavity bounded by the said platens.
 22. A process for the manufacture of a polymeric twin wall board as claimed in any one of claims 15 to 21 wherein the polymeric material used has a crystalline freezing point which is more than 120° C. and less than 150° C.
 23. A process for the manufacture of a polymeric twin wall board as claimed in claim 22 wherein the crystalline freezing point of the polymeric material is more than 125° C.
 24. A process for the manufacture of a polymeric twin wall board as claimed in any one of claims 15 to 23 wherein the polymeric material has a thermal conductivity greater than 0.20 W/m.K.
 25. A process for the manufacture of a polymeric twin wall board as claimed in claim 24 wherein the polymeric material has a thermal conductivity of between 0.25 to 0.35 W/m.K.
 26. A process for the manufacture of a polymeric twin wall board as claimed in any one of claims 15 to 21 wherein said polymeric material is a modified or unmodified polyolefin having a thermal conductivity of greater than 0.20 W/m.K and a crystalline freezing point more than 120° C. and less than 150° C., wherein the vacuum applied to at least one side of the extruded polymeric material is at least −150 mBar.
 27. A process for the manufacture of a polymeric twin wall board as claimed in claim 26 wherein the vacuum applied to at least one side of the extruded polymeric material whilst in the cavity is between −175 mBar and −300 mBar.
 28. A process for the manufacture of a polymeric twin wall board as claimed in any one of the previous claims wherein the vacuum applied is constant and the extrudate is held in place while the solidifying process takes place.
 29. A process for the manufacture of a polymeric twin wall board as claimed in any one of claims 15 to 28 further including the step of transferring the solidified product into a cavity between a further pair of platens where a vacuum in the order of −30 to −50 mBar is applied.
 30. A polymeric twin wall board substantially as hereinbefore described with reference to what is shown in any one of the drawings.
 31. A process for the manufacture of a polymeric twin wall board as hereinbefore described with reference to what is shown in any one of the drawings. 